Environmental continuous air monitor inlet with combined preseparator and virtual impactor

ABSTRACT

An inlet for an environmental air monitor is described wherein a pre-separator interfaces with ambient environment air and removes debris and insects commonly associated with high wind outdoors and a deflector plate in communication with incoming air from the pre-separator stage, that directs the air radially and downward uniformly into a plurality of accelerator jets located in a manifold of a virtual impactor, the manifold being cylindrical and having a top, a base, and a wall, with the plurality of accelerator jets being located in the top of the manifold and receiving the directed air and accelerating directed air, thereby creating jets of air penetrating into the manifold, where a major flow is deflected to the walls of the manifold and extracted through ports in the walls. A plurality of receiver nozzles are located in the base of the manifold coaxial with the accelerator jets, and a plurality of matching flow restrictor elements are located in the plurality of receiver nozzles for balancing and equalizing the total minor flow among all the plurality of receiver nozzles, through which a lower, fractional flow extracts large particle constituents of the air for collection on a sample filter after passing through the plurality of receiver nozzles and the plurality of matching flow restrictor elements.

The present invention generally relates to environmental continuous airmonitors, and, more specifically relates to environmental air monitorinlets used in adverse environmental conditions. This invention was madewith Government support under Contract No. W-7405-ENG-36 awarded by theU.S. Department of Energy. The Government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

Environmental continuous air monitors (ECAMs) are becoming much moreimportant to National defense programs in light of recent world events.The specter of the use of “dirty” nuclear devices or radiologicaldispersal devices (RDDs) by terrorists has become a real possibility. Tohandle the consequences of such attacks, high volume environmentalcontinuous air monitors must be reliable, and provide service over longperiods of time without requiring frequent maintenance. However, aproblem with current environmental continuous air monitor designs isthat sampled ambient air may contain large quantities of fine particles(particles in the size range less than about 2.5 micrometers aerodynamicdiameter) and smoke particles (particle diameters of between about0.1-1.0 micrometers), which can cause rapid filter clogging andsubsequent premature shutdown. This is especially true of ambientatmospheres in the aftermath of RDD explosions or associated fires. Ithas been found that in the case of field vegetation burning, forexample, that over 80% of the total suspended particulates have sizes of1 micrometer or less in diameter. It has long been recognized that fineand smoke sized particles cause rapid clogging of the pore structuresand, understandably, sampling at high volumetric rates accelerates dustloading of the sample filter and, where smoke or fine dusts areinvolved, filter clogging and sampling failure. This frequent shutdownand need for replacement of filters, is time consuming and expensive,and necessarily can interrupt an important monitoring and consequencemanagement process.

Fine particulates exist naturally in many environments as well as aresult of explosions. Among these are the very smoky conditions thatdevelop in the vicinity of forest and range fires where fires and smokeare principal constituents of the ambient aerosol load. Elsewhere,energetic disturbances of soil that can accompany earth moving andconstruction activities can create high dust loads containing a largefraction of fines, and smoke particles are copiously generated byinternal combustion engines, especially diesel engines. These conditionscan quickly cause a prior art environmental continuous air monitorrapidly to experience clogged filters and a subsequent shut down ofoperation.

The present invention addresses the problem of rapid plugging of filtersin high volume ambient continuous air monitors by selectively removingdebris and fine particle components in the sampled ambient air before itpasses through the sample collection filter, while at the same time,capturing the larger size particles that would be expected to containthe material of concern, such as the radioactive constituents in aradiological dispersal device (RDD) mixed in with fine dusts and smoke.

In order to achieve the objects and purposes of the present invention,and in accordance with its objectives, an inlet apparatus for removal ofmaterials that preclude sustained, low maintenance operation comprises apre-separator stage for eliminating suspended debris at the input to thesystem that might damage or degrade the performance of subsequent inletstages, followed by a multiple nozzle virtual impactor stage having a50% cut-point of about 2 micrometers aerodynamic diameter for removalwithout filtration of 90% or more of the fine particle and smokeconstituents in the sampled air. Extracted environmental air passingthrough the pre-separator stage often contains re-suspended debris(leaves, flying insects, etc) that can be removed by inertial separationtechniques to be deposited in traps, and then, upon exiting the throatof the first stage, passes over a deflector plate in a transition zoneinto a second stage consisting of a multiple nozzle dichotomous virtualimpactor. Here in a third stage, the fine fraction of aerosol areremoved and discarded without filtration, thereby achieving theobjective of reducing the rate at which filter performance degrades. Atthe same time, nearly all of the larger size particles are transportedto the filter and collected.

SUMMARY OF THE INVENTION

In order to achieve the objects and purposes of the present invention,and in accordance with its objectives, an inlet for an environmental airmonitor comprises a pre-separator stage interfacing with ambientenvironment air and removing debris and insects commonly associated withhigh wind outdoors with a deflector plate in communication with incomingair from the pre-separator stage, that directs the air radially anddownward uniformly into a plurality of accelerator jets located in amanifold of a virtual impactor, the manifold being cylindrical andhaving a top, a base, and a wall, with the plurality of accelerator jetsbeing located in the top of the manifold and receiving the directed airand accelerating directed air, thereby creating jets of air penetratinginto the manifold, where a major flow is deflected to the walls of themanifold and extracted through ports in the walls. A plurality ofreceiver nozzles are located in the base of the manifold coaxial withthe accelerator jets, and a plurality of matching flow restrictorelements are located in the plurality of receiver nozzles for balancingand equalizing the total minor flow among all the plurality of receivernozzles, through which a lower, fractional flow extracts large particleconstituents of the air for collection on a sample filter after passingthrough the plurality of receiver nozzles and the plurality of matchingflow restrictor elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate an embodiment of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a cross-sectional view of the present invention including arepresentative pre-separator mated with the virtual impactor section.

FIG. 2 a is a perspective illustration of the virtual impactor sectionof the present invention.

FIG. 2 b is a cross-sectional view the virtual impactor section of thepresent invention.

FIGS. 3 a, 3 b, and 3 c show various views of the receiver nozzles ofthe virtual impactor of the present invention.

FIGS. 4 a, 4 b, and 4 c show various views of the flow restrictorelements of the virtual impactor of the present invention.

DETAILED DESCRIPTION

The present invention provides apparatus that is positioned in theairflow path of a high volume environmental continuous air monitor aheadof the sample collection stage to remove potentially interfering andclogging particles in the collected air. The invention may be understoodmost easily through reference to the drawings.

Referring first to FIG. 1, a cross-sectional view of one embodiment ofthe present invention is shown. Pre-separator 11 may be any of numerousacceptable pre-separators, but in FIG. 1 a cyclo-shroud typepre-separator (U.S. Pat. No. 6,530,287) is shown. The requirements ofpre-separator 11 are that it buffer the inlet of a continuous airmonitor against the sometime high velocity, dust-laden ambient air inputthrough inlets 11 a. The resultant inertial and drag effects on piecesof airborne debris or insects that are carried into the inlet and thensubjected to strong cyclonic or centrifugal forces, causes such debristo be removed and trapped before it can move on into the filter of acontinuous air monitor.

In practice, the design of the pre-separator 11 should be such that onlyparticulate matter that is about 10-15 micrometers aerodynamic diameteris passed on to the next stage of the apparatus. Inasmuch as the presentinvention is a two-stage device consisting of a pre-separator stagefollowed immediately by dichotomous virtual impaction separation stage12, a transition region 11 b exists between stages where the outlet flowfrom the pre-separator 11 is diverted radially and then down uniformlyinto the input level of virtual impactor stage 12.

As shown, deflector plate 13 causes the output flow of the cyclone orother pre-separator 11 to deflect, redistribute, and thereby enter thevirtual impactor stage 12 in a uniform fashion. Without defector plate13, the output from transition region 11 b of pre-separator 11 wouldimpinge directly on the center section of virtual impactor stage 12where turbulence may cause loss of particulate sample, and fluctuatingor non-uniform flow patterns to develop over accelerator jets 14.However, with deflector plate 13 in place, the deflected air can entersaccelerator jets 14. Accelerator jets 14 feature a flow-straighteningcollar that causes the airflow to enter accelerator jets 14 smoothly andflow straight into the jet orifices and on into manifold 16. As is knownfrom prior art, it is the diameter of these orifices that principallydetermines the 50% cut-point diameter of the impactor, which in thepresent application is approximately 2 micrometers aerodynamic diameter.

The major portion (typically 90%-95%) of the total flow exits frommanifold 16 through exit ports 16 a. This major flow contains most ofthe smaller, filter clogging, particles, and exhausted through port 17 ain casing 17 by way of exhaust piping (not shown).

A minor portion (typically 5%-10%) of the total flow into manifold 16 isextracted through the flow restrictor elements l8 aof receiver nozzles18, and exits through flow restrictor elements 18 a. The function offlow restrictor elements 18 a is to provide a means to balance the minorflow evenly among the plurality of receiver nozzles 18. Each receivernozzle 18 assembly faces a corresponding accelerator jet 14, and has aninitial bore diameter only somewhat larger than the jet orificediameter, but which expands to a much larger diameter. This largerdiameter expansion section terminates at the point of insertion of aflow restrictor element having a tapered bore starting at the diameterof the expansion section and reducing to a much smaller bore, sized tocreate sufficient differential pressure to balance flow between theplurality of receiver nozzles 18, and shaped, along with flow restrictorelements 18 a, to smoothly carry the separated larger particles in theminor flow through to a sample filter collection stage in the associatedcontinuous air monitor with a minimum amount of wall deposition. Theouter diameter of the restrictor elements 18 a can be threaded (e.g., ¾NF-16) to match a corresponding inner thread in receiver nozzles 18 sothat restrictor elements 18 a can be assembled and disassembled formaintenance and cleaning. This will be more fully described below.

FIG. 2 a is a perspective view of the virtual impactor portion of thepresent invention and is better illustrated in a cross-sectional in FIG.2 b, and having the same numbers. In particular, FIG. 2 b betterillustrates the nature of accelerator jets 14 and receiver nozzles 18with flow restrictor elements 18 a attached, and with major flow exitports 16 a that allow the exhaust of the major flow.

The minor portion of the sampled air exiting the receiver nozzles 18 andflow restrictor elements 18 a, containing the large particle load of thesampled air then is drawn down and through the sample collection filterof the continuous air monitor instrument for determination of whetherdangerous materials are present. The minor flow volumetric rate istypically one-fifth to one-tenth of the total inlet sampling rate (e.g.,0.5-1.0 CFM out of 10 CFM intake). To illustrate, it has been found thata combination of 8 accelerator jets 14/receiver nozzles 18 pairs can beeffectively used at total flow rate of 10 CFM (minor flow of 1 CFM) toachieve a cut point of approximately 2 micrometers AD. For otherapplications and higher flows, a different number of accelerator jets14/receiver nozzles 18 would be necessary.

Referring now to FIGS. 3 a, 3 b, and 3 c, receiver nozzle 18 is shownhaving an upper section illustrating a small diameter inlet bore,expanding to a larger bore such that wall deposition loss is minimized,and terminating in a threaded outlet to mate with a flow restrictionelement. In FIGS. 4 a, 4 b, and 4 c, flow restriction 18 a is shown,with inner diameter tapering from the expansion section of receivernozzle 18 upper section down to an outlet diameter selected to createsufficient pressure drop that flows through all nozzles in amulti-nozzle array are uniform. The outer diameter of restrictionelement 18 a can be threaded so that it will screw into the threadedbase of a receiver nozzle 18.

The present invention, in addition to removing airborne debris as wellas fine soil particles or smoke accompanying high wind in the openenvironment, also provides a benefit for environmental continuous airmonitors designed to detect radioactive aerosols released to outdoorair. Since the particulate progeny arising from the decay of radon gas(i.e., atoms of Pb, Bi, and Po) are of molecular size or preferentiallyattached to fine dust particles, they would be separated and discardedalong with smoke and other fine particles in this invention. This wouldhave the effect of reducing the background count rate in radiationdetectors used in radiological environmental continuous air monitors andimprove performance. Finally, although clogging of environmentalcontinuous air monitor filters is not totally eliminated with thepresent invention (some fine particles are invariably associated withthe minor flow), the time between filter changes is considerablyreduced.

It is the ability of the present invention to both effectively removedebris and coarse particles in ambient air for protection of subsequentstages of the apparatus, and effectively remove fine particles and smoketo protect the sample filter from premature failure that makes theinvention so important to environmental continuous air monitoring.Although the sample filter is filtering air at a reduced rate(approximately 0.5-1 CFM), it is receiving nearly the entire quantity ofsuch particles collected at 10 or 20 CFM, and thus the sensitivity fordetection is correspondingly increased.

The foregoing description of the embodiments of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical application to thereby enable others skilled in the art tobest utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

1. An inlet for an environmental air monitor comprising: a pre-separatorstage for interfacing with ambient environment air and removing debrisand insects commonly associated with high wind outdoors; a deflectorplate in communication with incoming air from said pre-separator stage,for directing said air radially and downward uniformly into a pluralityof accelerator jets located in a manifold of a virtual impactor, saidmanifold being cylindrical and having a top, a base, and a wall, withsaid plurality of accelerator jets being located in said top of saidmanifold and receiving said directed air and accelerating said air,thereby creating jets of said directed air penetrating into saidmanifold, where a major flow is deflected to said walls of said manifoldand extracted through ports in said walls, and; a plurality of receivernozzles located in said base of said manifold coaxial with saidaccelerator jets, and a plurality of matching flow restrictor elementsin the plurality of receiver nozzles for balancing and equalizing thetotal minor flow among all the plurality of receiver nozzles, throughwhich a lower, fractional flow extracts large particle constituents ofsaid air for collection on a sample filter after passing through saidplurality of receiver nozzles and said plurality of matching flowrestrictor elements.
 2. The apparatus as described in claim 1, whereinsaid pre-separator apparatus consists of a cyclone inlet.
 3. Theapparatus as described in claim 1, wherein said pre-separator consistsof a baffled inlet stage.
 4. The apparatus as described in claim 1,wherein said plurality of accelerator jets are composed of eightaccelerator jets.
 5. The apparatus as described in claim 1, wherein saidplurality of receiver nozzles and said plurality of flow restrictorelements are composed of eight receiver nozzles and eight flowrestrictor elements.
 6. The apparatus as described in claim 1, whereinsaid plurality of flow restrictor elements have exterior threads to matewith interior threads in said plurality of receiver nozzles, enablingsaid plurality of flow restrictor elements to be removed for cleaning.7. The apparatus as described in claim 1, wherein said pre-separator andsaid deflector plate are circular in shape and positioned to achieveuniform, radially distributed air from said pre-separator stage intosaid impactor stage.